The concepts of controlled-release fertilizers and of the use of sulfur coatings to impart the controlled-release characteristics to granules or particles of fertilizer are known. It is recognized, for example, that slowing the release of plant nutrients such as nitrogen from highly soluble fertilizer granules is desirable because releasing the nutrients over an extended period of time achieves advantages which include increased efficiency of fertilizer use by plants, reduction in cost by requiring fewer applications of fertilizer and reduction in nutrient losses caused by backing and denitrification. It is also recognized that application of a thin coating of sulfur on the fertilizer granules reduces the dissolution rate of the granules and hence imparts controlled-release characteristics. In essence, the water in the soil, and rain water are kept away from the very soluble fertilizer until a granule develops a flaw such as a crack or fissure in the coating, or the coating is penetrated by microbial action. When the fertilizer is initially put on the soil, some of the granules already have flaws which water can attack through capillary action and absorption for poorly covered or coated granules. Granules applied to the soil fail at different rates because the granules are not uniformly coated either from the standpoint of a specific granule or between granules. Thus, the time of failure of a granule and release of its nitrogen to the soil is different for different granules. Sulfur coated fertilizer, therefore, has its slow release characteristics based on the statistical pattern of failure for the granules.
In considering sulfur coated urea, for example, most of these products are made with not only a sulfur coating but also coatings of a wax or wax-like substances and a conditioner (a finely divided absorbent powder such as diatomaceous earth). The sulfur alone in these products gives very little slow release, rather, it acts as a prime coat which is then sealed by the wax. The conditioner gives the product free flowing characteristics and prevents sticking. These last two coatings, however, perform an additional function; they provide a cushioning effect to the granules and prevent the sulfur coating from cracking severely under normal bulk handling. If the coating does crack, a good sealant will flow into the crack and seal it during storage. Thus, some sulfur coated urea materials improve with storage.
There are different products, however, also known as sulfur coated urea. They contain no wax or conditioner and are commonly known as "Sulfur-Only" sulfur coated urea. The present invention provides an improvement to this kind of product. The process of the invention may also be of value in the conventionally sealed sulfur coated coated urea. For many years it was felt that such a sulfur-only product could not be made with an economically feasible amount of sulfur and still have good slow release properties. This was proven wrong by continued development efforts, however, the resulting products are very fragile, especially after the sulfur is allowed to age for several hours. Quick cooling and proper temperature control of the sulfur coated urea during production maximizes the elastic (amorphous) sulfur in the sulfur coating of the final product and gives the coating some minor resistance to breakage. Therefore, with normal handling, the coating cracks and the slow release characteristics are in large part destroyed. Granules with cracked coatings might slow the release of the urea and somewhat prevent burning of plants, but will not give the extended release over weeks and months which is desired for sulfur coated urea. Sulfur-only products are, nevertheless, superior to other coated fertilizers in some ways in that (1) they require much less equipment and manpower to make, (2) they are not dusty as they have no conditioner, and (3) they are not sticky as they have no wax.
The primary object of the present invention is to improve the bond between the sulfur coating and the fertilizer granule and to improve the strength of the sulfur coating, with the result that the coated granules have a reduced tendency to break or crack during storage and during normal handling. The coated granules therefore have more uniform controlled-release properties than prior sulfur-only coated granules, and the controlled-release properties are improved in the sense that nutrients are released over a longer time period than for prior sulfur-only coated granules.
According to the principles of this present invention these advantages are obtained by providing a special bonding layer between the inner core of fertilizer and the external sulfur coating, the bonding layer containing discrete but interwoven crystalline portions of sulfur and fertilizer with the fertilizer portion of the bonding layer being attached to the crystalline structure of the core and the sulfur portion of the bonding layer being attached to the crystalline structure of the subsequently applied sulfur coating. The sulfur coating and the fertilizer core are thus mechanically interwoven and fused together.
The bonding layer, to be effective, should be co-applied in the sense that the fertilizer component and the sulfur component of the layer are applied to the core simultaneously in order to ensure that the above-described mechanical bonds are obtained. This is best achieved by simultaneously spraying the two components in molten form onto the cores, either as separate sprays or as a mixed spray. Accordingly, the present invention includes the process of coating fertilizer particles by first co-spraying the particles with molten droplets of sulfur and fertilizer and then applying a coating of sulfur, also preferably by a melt-spraying technique.
The applicant is not aware of any prior teachings which are relevant to the above-summarized concepts of a special intermediate bonding layer for holding a sulfur coating to a fertilizer particle and of melt-spraying such intermediate bonding layer. The applicant is aware, however, of certain U.S. patents which relate to conventional fertilizer products and processes. U.S. Pat. No. 3,295,950 teaches the application of a coat of waxes and/or oils to fertilizer pellets as an undercoating, followed by a coating of sulfur and finally a sealant coating of the wax and/or oil.
U.S. Pat. Nos. 3,877,415 and 3,991,225 disclose an apparatus for spraying coating materials onto a continuous cascade of solid particles and a process for forming coated particles.
U.S. Pat. No. 3,903,333 discloses a process for sequentially applying sprays of coating materials (such as sulfur) to solid particles (e.g. urea). U.S. Defensive Publication No. T912,014 discloses a process for applying precisely uniform solid coatings onto solid particles by sequentially spraying the coating material onto the particles.
U.S. Pat. 3,576,613 discloses an improvement to the "contact" angle between sulfur and urea at the coating interface in sulfur coated particles, further involving the addition of a finely divided powder, in particular, carbon black, to the surface of the particle substrate.
U.S. Pat. No. 4,330,319 discloses a process for mixing molten urea and sulfur to produce solid particles of the mixture (No coating is involved).
None of the above documents suggest the novel features of the present invention.